The nature of CDMA, which requires a high peak power to average power ratio of 8 dB or higher, makes providing a linear RF path of constant gain, a difficult and costly challenge. Since base station cost is RF amplifier-centric, failure to accurately regulate the RF path either wastes amplifier power or under utilizes the amplifier capacity. The base station cost in terms of size, weight, heat, and direct current (DC) power, is directly affected by the method of regulating the RF path gain.
Furthermore, preserving the nominal path gain is important to maximizing service provider revenue and mobile service availability. If path gain is calibrated too low, the RF amplifier power overload protection acts too soon at a lower air interface call capacity, which implies lower revenue for the service provider and disappointed customers unable to obtain service on demand. For example, if due to losses, the path gain was 1 dB lower than expected, a call that would normally require a gain setting of 60 digital gain units (“dgu”) (a unit of voltage) would require 67 dgus to provide the mobile with the same signal power, measured in dgu2 (e.g., 1.259*602˜=672, where 1 dB˜=1.259). Since the maximum power is nominally 77760 dgu2, about seventeen 60 dgu calls could be supported (e.g., 77760−16800 overhead˜=17*60*60) where as, only about thirteen, 67 dgu calls could be supported (e.g., 77760−16800 overhead˜=13*67*67). Therefore, while the system would try to support seventeen calls, it would be doing so at 67 dgu per call. The RF amplifier overload protection would kick in at about thirteen calls, supporting roughly only three quarters of the number of calls that should be supported.
If, on the other hand, path gain is calibrated too high, equipment life may be shortened and as described below, the FCC can take regulatory action against the service provider for spectral non-compliance. Indeed, a high path gain may cause too much DC current in the power amplifier, blowing the power supply fuse and shutting down the RF path through that amplifier. A high path gain also distorts the RF coverage footprint, which increases interference to neighboring cells and lowers the network air interface capacity. This occurs because: (i) path gain is applied equally to the traffic signals and the pilot signal. The pilot signal is used by mobile stations to determine the base station they are to listen to for their signal. With high path gain, the pilot signal will be too strong, thereby attracting too much traffic away from neighboring cells; (ii) the amplifier is operated at a higher than expected power (e.g., If the RF path gain increased 1 dB higher than the nominal path gain, a 60 dgu call would only need 54 dgu to provide the mobile with the necessary power (e.g., 602/1.259˜=542, where 1 dB˜=1.259). The cell would therefore accept and attempt to support approximately 20 calls, as opposed to 17, before amplifier overload protection kicks in. This in turn could introduce noise and out-of-band emissions. Mobile stations would then require more signal power to overcome the noise and FCC out-of-band emission power limits could be exceeded.
Present art provides individual solutions for each constraint that affects path gain such that they generally do not account for interactions among components in the RF path. Furthermore, most of the solutions are static and non-tunable. These include: using additional semiconductor junctions to improve linearity in each amplification stage over the operational temperature range; matching component parts by selection, which requires extra steps in the manufacturing process; laser trimming of components until their properties satisfy the specifications, which also requires extra production steps; and connecting passive components with inverse thermal characteristics in series to constrain path gain variance over temperature.
Furthermore, current solutions use wide tolerance ranges for path gain, requiring larger and more costly equipment than would be required with tighter tolerances. Much control is essentially open loop, providing no feedback means for adjusting the path gain based on actual performance of the path. Rather, conservative nominal values are assigned to constraints, a priori. Manual adjustment is often required to optimize performance, which only remains optimal over a narrow temperature range.
There are also closed loop control solutions that measure input and output power and adjust the path gain to preserve the nominal gain, which is the desired ratio of output power over input power. This is done within components as well as across a plurality of components.
Accordingly, it is desirable to preserve a nominal RF path gain seamlessly and accurately by compensating for gain changes in multiple physical components, which may be non-linear, without exceeding their physical constraints and in a way that provides for performance tuning and product evolution.